Add petab.v2.Problem

[dweindl]

Copy of v1.Problem with all deprecated functionality removed.

Loading PEtab 2.0 problems with petab.v1.Problem is now deprecated.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 53.96825% with 87 lines in your changes missing coverage. Please review.

Project coverage is 74.37%. Comparing base (721c0ed) to head (6dc222f).

Files	Patch %	Lines
petab/v2/problem.py	53.26%	80 Missing and 6 partials ⚠️
petab/v1/problem.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #285      +/-   ##
===========================================
- Coverage    75.25%   74.37%   -0.89%     
===========================================
  Files           69       70       +1     
  Lines         4260     4448     +188     
  Branches       900      952      +52     
===========================================
+ Hits          3206     3308     +102     
- Misses         789      869      +80     
- Partials       265      271       +6     

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[dilpath]

👍

[dilpath]

Were to_files and to_files_generic deprecated / Is there a method to write a PEtab problem to disk, in petab.v2?